I'm a bit hit and miss with telly shows, I almost never see them on first run and usually end up downloading series, streaming episodes when I remember to, or buying boxed sets. The stuff I like enough to watch regularly or buy on DVD is shorter than I thought it would be.

Stuff I own:
House of Cards trilogy
Firefly
Band of Brothers
Dinnerladies
The Royle Family
Early Doors
Blackpool

Stuff I like(d) enough to watch properly, on first showing or on their repeat showings on various Sky channels:
Q.I.
Gilmore Girls (Shakes head vigorously, to avoid catatonia induced by excessive contemplation of Lauren Graham)Doctor Who
Kingdom (BOOO to ITV for cancelling it. )
Sharpe
The Sweeney
Jeeves & Wooster
Red Dwarf
Buffy
Angel
The Muppet Show
Dungeons & Dragons
Just about any Cosgrove Hall animation from the 80's or early 90's. Duckula, Dangermouse, Victor & Hugo, The Wind in the Willows, Truckers, Chorlton and the Wheelies, Jamie and the Magic Torch...

-EDIT- I forgot to mention the only series I'm currently watching, Star Wars: The Clone Wars. The writing is corny, the voice acting is cheesy, the animation is a bit cheap looking, but... you know... Star Wars!

Life on Mars - I was growing up in the 70s, so even as it plays in Great Britain, I still find many loving memories in that series..

Heroes - switching the characters from hero to villian and back again is really nice.. many holes in the plot, but it's supposed to be fun, not science..

Taking the Flak - Just awesome funny.. and wonderfully political incorrect

Spooks - Awesome, too.. way better than any US series about secret service activities..

Burn Notice - that's a good US secret service series.. although it's more of an action comedy..

and among so many others were the Blackadders, Primeval, Torchwood, Monty Python (if you could call that a series..)

What I'd love to see - but it was never shown on german TV - is 'allo 'allo
Had a glimpse at one episode many years ago, while visiting a friend who was working for the british army in germany. Of course I could get me the series on DVD - but it's about 120,- Euros and I'm not sure if I would want to risk that money on a one episode impression...

And, away from the thread's title, here's my candidate for "worst series EVER by far" - Harper's Island
The plot is absolutely unrealistic and without any trace of logic...